PHOTOSYSTEM-II IN A MUTANT OF CHLAMYDOMONAS-REINHARDTII LACKING THE 23 KDA PSBP PROTEIN SHOWS INCREASED SENSITIVITY TO PHOTOINHIBITION IN THE ABSENCE OF CHLORIDE

The psbP gene product, the so called 23 kDa extrinsic protein, is invo lved in water oxidation carried out by Photosystem II. However, the pr otein is not absolutely required for water oxidation. Here we have stu died Photosystem II mediated electron transfer in a mutant of Chlamydo monas reinhardtii, the FUD 39 mutant, that lacks the psbP protein. Whe n grown in dim light the Photosystem II content in thylakoid membranes of FUD 39 is approximately similar to that in the wild-type. The oxyg en evolution is dependent on the presence of chloride as a cofactor, w hich activates the water oxidation with a dissociation constant of abo ut 4 mM. In the mutant, the oxygen evolution is very sensitive to phot oinhibition when assayed at low chloride concentrations while chloride protects against photoinhibition with a dissociation constant of abou t 5 mM. The photoinhibition is irreversible as oxygen evolution cannot be restored by the addition of chloride to inhibited samples. In addi tion the inhibition seems to be targeted primarily to the Mn-cluster i n Photosystem II as the electron transfer through the remaining part o f Photosystem II is photoinhibited with slower kinetics. Thus, this mu tant provides an experimental system in which effects of photoinhibiti on induced by lesions at the donor side of Photosystem II can be studi ed in vivo.